Chapter 27 Metabolic disorders
ELECTROLYTE EMERGENCIES
Theory
• Serum electrolyte concentrations are usually tightly controlled unless excessive gains or losses overwhelm homeostatic balance.
• Electrolyte balance requires normal hormonal control of intact kidney function. Primary renal dysfunction or the effects of drugs or endocrine disease can result in abnormalities.
Causes/assessment
• Electrolyte losses: renal (drug effects or endocrinopathy), GIT (vomiting, diarrhoea, surgical drains), skin (sweating, burns)
Clinical effects
• Electrolytes have a role in establishing the membrane potential, receptor signalling and enzyme function.
Management
Treat the patient, not just the abnormality.
• Severe, life-threatening or resistant cases, e.g. neurological (seizures, coma), CVS (hypotension, abnormal ECG), respiratory muscle failure (hypoventilation, tetany)—get input from the intensive care unit (ICU).
(See Table 27.3 for a list of electrolyte abnormalities, clinical indications and treatment.)
ACID–BASE DISTURBANCES
Pearls/pitfalls
• An acute derangement in acid–base balance invariably signifies a significant disease process (most patients need admission or at least extended observation).
• An abnormal or recent change in venous bicarbonate may be the first clue to an acid–base disturbance.
Determining the acid–base abnormality
Metabolic acidosis
(Mild: [HCO3–] < 18 mmol/L; moderate: < 15 mmol/L; severe: < 12 mmol/L)
• Important causes of wide anion gap metabolic acidosis: poor tissue perfusion, hypovolaemia and/or sepsis (most common), diabetic ketoacidosis (DKA), renal failure, drug poisoning.
• Less common causes: bicarbonate loss from GUS or GIT/‘normal anion gap metabolic acidosis’ e.g. renal tubular acidosis, surgical fistula/stomal/drain losses, Addison’s disease.
• Rapidly assess the following—circulatory status (HR, BP, urine output), infection (fever), hyperglycaemia (bedside blood sugar level (BSL), urinary ketones).
• Important lab tests—electrolytes, anion gap, renal function, serum osmolarity, serum ketones, BSL, WCC.
Metabolic alkalosis
(Mild: [HCO3–] > 25 mmol/L; moderate: > 30 mmol/L; severe: > 35 mmol/L)
• Common causes in emergency department include: diuretic use, gastric losses (e.g. pyloric stenosis, bulimia nervosa).
Respiratory acidosis
(Mild: pCO2 > 45 mmHg; moderate: > 55 mmHg; severe > 65 mmHg)